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Photons may convert to axion-like particles (ALPs) in external magneticfields. Under certain conditions, this effect should result in irregularfeatures in observed spectra of astrophysical sources. Lack of suchirregularities in particular spectra was used to constrain ALP parameters, withtwo most popular sources being the radio galaxy NGC 1275 and the blazar PKS2155-304. The effect and, consequently, the constraints, depend on the magneticfields through which the light from the source is propagated. Here, we revisitALP constraints from gamma-ray observations of NGC 1275 taking into account theregular magnetic field of the X-ray cavity observed around this radio galaxy.This field was not accounted for in previous studies, which assumed a model ofpurely turbulent fields with coherence length much smaller than the cavitysize. For the purely regular field, ALP constraints are relaxed considerably,compared to the purely turbulent one. While the actual magnetic field around asource is an unknown sum of the turbulent and ordered components, thedifference in results gives an estimate of the theoretical uncertainty of thestudy and calls for detailed measurements of magnetic fields around sourcesused to constrain ALP properties in this approach.

physics letters. b

On the impact of magnetic-field models in galaxy clusters on constraints  on axion-like particles from the lack of irregularities in high-energy  spectra of astrophysical sources